Europe migrant crisis: Dozens die in hold of Libya boat

The accounts of Mueller's ordeal were first reported by British newspaper The Independent and later confirmed by her family to the Associated Press and ABC News.

Mueller, 26, from Prescott, Ariz., was taken captive in Syria in August 2013 while leaving a Spanish Doctors without Borders hospital in Aleppo.

Al-Baghdadi, the self-proclaimed "caliph" of the Islamic State, brought her "live and in person" to the home of Abu Sayyaf, a Tunisian in charge of oil and gas revenue for the group, counter-terrorism officials have told ABC News over the past several months.

The details of Mueller's treatment were initially reported by several Yazidi girls who were held at the house, including a 14-year-old and her sister who managed to escape in August 2014, The Independent reported. The teen's version has been corroborated by U.S. officials.

The Yazidi teen eventually made her way to Iraqi Kurdistan, where she talked to U.S. commandos in November 2014, the Associated Press reported. Intelligence agencies corroborated her account and American officials passed it on to Mueller's parents in June.

Some 320 others were rescued when the vessel was intercepted off Libya.

The dead were found in the fishing boat's hold. It is thought they died after inhaling fumes from the engine, the rescue vessel's captain said.

European officials say the plight of migrants, almost 250,000 of whom have crossed by boat to the continent this year, is "beyond urgent".

So far this year, more than 2,000 migrants have died trying to cross the sea to Europe, the UN says.

The BBC's James Reynolds in Rome says it has become the world's most dangerous migrant journey.

Distressing

The fishing boat was apparently taking on water when it was spotted about 21 nautical miles (39km) off the Libyan coast and south of the Italian island of Lampedusa.

Interior Minister Angelino Alfano said about 320 people had been rescued but "they are still counting the victims". Those rescued included about 10 women, as well as children.

The commander of the naval vessel involved in the rescue said his officers had encountered distressing scenes when they boarded the fishing boat.

They found dead bodies in the hold floating in water, fuel and human excrement, Massimo Tozzi said.

In other developments in the eastern Mediterranean:

Coastguards on the Greek island of Kos said they rescued more than 200 people in several small boats on Saturday, Reuters reportedA ferry sent to Kos to be a floating registration and accommodation centre for 2,500 refugees was not yet operational 24 hours after arriving.